| Work Detail |
Scientists in the United States have designed a microwire solar cell that could reportedly enable the coupling of singlet fission with silicon. Key to their achievement was an interface that transfers the electrons and holes sequentially into silicon instead of both at once. Working with an effect known as singlet exciton fission (SF), scientists from the Massachusetts Institute of Technology (MIT) have demonstrated a novel silicon solar cell concept that could potentially surpass the quantum efficiency limit for conventional PV devices. Singlet exciton fission is an effect seen in certain materials whereby a single photon can generate two electron-hole pairs as it is absorbed into a solar cell rather than the usual one. The effect has been observed by scientists as far back as the 1970s and though it has become an important area of research for some of the world’s leading institutes over the past decade; translating the effect into a viable solar cell has proved complex. Singlet fission solar cells can produce two electrons from one photon, making the cell more efficient. This happens through a quantum mechanical process where one singlet exciton (an electron-hole pair) is split into two triplet excitons. “Up to now, we have only had indirect evidence that is possible to couple singlet exciton fission to silicon,” the researchs corresponding author, Marc A. Baldo, told pv magazine. “The breakthrough for us was to design an interface that transfers the electrons and holes sequentially into silicon instead of both at once.” In the study “Exciton fission enhanced silicon solar cell,” which was recently published in Joule, the researchers explained that they designed and built a microwire (MW) cell with an interface based on a hafnium oxy-nitride (HfOxNy) film to improve the coupling between tetracene (Tc) and silicon. Tc and its derivatives are prime candidates for SF, as they can form charge transfer and multi-excitonic states. The interface also included a thin aluminum oxide (AlOx) passivation layer that prevents the transferred charge carriers from immediately recombining at the silicon surface, as well as a zinc phthalocyanine (ZnPc) layer as an electron-donor material. “To minimize recombination on the rear side, a back surface field (BSF) layer with a junction depth of 1 µm and a localized back contact is added,” the scientists said. “A microgrid electrode is applied as the front electrode to efficiently collect carriers.” The researchers conducted a series of measurements on the cell performance and found that depositing ZnPc and Tc on the device enchanges the short-circuit current density, with negligible decrease in the open-circuit voltage and fill factors, resulting in an overall enhancement in power conversion efficiency. The analysis also showed that the peak charge generation efficiency per photon absorbed in tetracene is around 138%, which the scientists said “comfortably” exceeds the quantum efficiency limit for conventional silicon solar cells. “This technology will compete with double junction concepts like perovskites on silicon,” Baldo explained. “Combining exciton fission with silicon avoids current matching constraints and the approach promises the robustness under varying illumination and simplicity typical of single junctions. It still has a long way to go. Most importantly, we need to increase efficiency and prove that the technology can be stable in sunlight.” “Observing photocurrent from exciton fission in a silicon solar cell is a proof of the concept that coupling to singlet exciton fission is a viable path to increasing the efficiency of silicon solar cells,” Baldo concluded. “I think we can now claim that exciton fission is a genuine technological contender in the competition for new solar cell technologies.” In 2023, researchers from MIT and the University of Virginia announced plans to use acenes, which are benzene molecules with unique optoelectronic properties, in singlet fission solar cells. Their approach consisted of adding carbodicarbenes ligands to acenes that were already doped with boron and nitrogen. In 2019, another MIT research group demonstrated how singlet exciton fission could be applied to silicon solar cells and could lead to cell efficiencies as high as 35%. They claimed to be the first group to transfer the effect from one of the ‘excitonic’ materials known to exhibit it, in that case also tetracene. They achieved the feat by placing an additional layer just a few atoms thick of hafnium oxynitride between the silicon solar cell and the excitonic tetracene layer. The MIT researchers described their work as “turbocharging” silicon solar cells and said it differs from the most common approaches to increasing solar cell efficiencies, which these days are focused more on tandem cell concepts. “We’re adding more current into the silicon as opposed to making two cells,” they stated at the time. |
